1. Field of the Invention
This invention relates to a mixing and dispensing apparatus for foamable compositions. More particularly, this invention relates to an apparatus and a method for blending and dispensing the ingredients of foamable, curable organosiloxane compositions. The cured foam is soft, flexible and exhibits a uniform distribution of small diameter cells. The present apparatus and method are particularly useful for preparing small diameter elongated articles such as foamed-in-place gaskets.
2. Background Information
Foamable organosiloxane compositions that generate hydrogen gas as a blowing agent during curing of the composition are known in the art, and are described in U.S. Pat. Nos. 3,923,705; 4,599,367; 4,888,217; and 5,079,292.
The prior art also describes numerous devices for blending two-part compositions containing a thermosetting organic or organosilicone polymer with a compressed gas or a chemical blowing agent in a confined area and dispensing the resultant mixture into the atmosphere where the entrapped gas forms a foam as the composition cures. Devices of this type are described in U.S. Pat. Nos. 4,778,631, which issued to Cobbs et al. and 5,082,142, which issued to Saidman et al. on Jan. 21, 1992. The Saidman et al. patent teaches dissolving the blowing agent in the one of the two parts of the composition and blending the two parts in a static mixer immediately prior to dispensing the composition from the apparatus.
The Saidman et al. patent does not require a particular nozzle configuration at the output of the static mixer to produce a useful foam.
An article by C. Rauwendaal entitled "A Guide To Static Mixers" (Plastics World, May 1992, pages 63-66) defines static mixers as devices designed to "split and reorient the flow and, thus, impart improved distributive mixing to the fluid". This article recommends mixers with a length to diameter ratio of at least 10 to achieve adequate mixing and product uniformity using the most efficient static mixers described in the article. This mixers are identified as type SMX manufactured by Sulzer and type ISG manufactured by Ross.
Rauwendaal considers static mixers containing the Kenics type of mixing element simple and easy to clean, but less efficient. Mixers containing these elements require a length to diameter ratio of at least 29 to achieve efficient mixing.
The present inventor discovered that when a static mixer is used, the diameter of the orifice through which the composition passes when exiting the device is critical to preparing a foam exhibiting uniform cells that are preferably no larger than two millimeters and contain a low percentage, typically less than 10 percent, of cells with ruptured walls. The present method and associated apparatus are based on this discovery. In accordance with the present invention, nozzles with an exit orifice smaller than 1.50 mm permit use of a static mixer containing Kenics type elements with a lower length to diameter ratio (11) than taught by the art, including the aforementioned article by C. Rauwendaal, which recommends a length to diameter ratio of at least 29 to achieve product uniformity using this type of mixing element.